The present invention relates to air conditioning systems, and in particular to heat pump systems of the absorption type in which naturally available liquid or gaseous fuel is used to generate vapor phase working fluid.
In consideration of the shortage of fuel and its rising cost, energy savings are of primary concern to designers of power plants for industrial or residential use. Since household air conditioning systems account for a substantial part of the total energy consumed by household appliances, the development of energy saving air conditioning systems, such as heat pumps, is of the utmost importance.
Heat pumps are generally classified under categories of the type in which the pressurized refrigerant is generated by an electrically driven compressor and the type in which the pressurized refrigerant is generated by a thermally driven source. Since the latter allows for highly efficient utilization of naturally available energy such as petroleum or gas rather than electric power which requires conversion from the natural energy source, attention is currently centered on the thermally driven type heat pump. Among the thermally driven type heat pumps the absorption type heat pump is preferred for household use because of its favorable characteristics in low maintenance and low noise.
In contrast with the electrically driven heat pump in which the evaporation temperature varies as a function of outdoor temperature, the absorption type heat pump is not capable of varying its evaporation temperature as a function of outdoor temperature so that the former is lower than the latter to extract heat from the environment because the evaporation temperature is determined solely by the absorption temperature of the solution in the absorber. The absorption type heat pump is thus not capable of operating satisfactorily in the heating mode when the outdoor temperature falls below the evaporation temperature.
The refrigerant concentration of the working fluid and its flow rate, if determined to achieve maximum efficiency for cooling operation, are inadequate for heating operation to permit the system to operate in a wide range of outdoor temperatures because the evaporation temperature is not below the outdoor temperature during cold weather. On the other hand, if the concentration is selected so that the evaporation temperature is always below the outdoor temperature during cold weather, frost is likely to form on the evaporator to thereby cause inefficiency during heating operation and the evaporation temperature is at a level too low causing inefficient cooling operation during hot weather.